I feel your pain OP.
My MIL and FIL are retired but not elderly. Generally I get on brilliantly with them, they're fab grandparents and nice people.
DH works out of home full time and I'm a SAHM to one preschooler and a school age child. I've started volunteering on the days my youngest is at preschool and I had a six week training course that took place on one of the days when youngest was home for two hours in the morning. No problem they said, we will come and watch her for you, go on your course.
So they arrive for 9:30 I leave and come back at 11:30. Make a cup of tea, happy to have a chat for an hour or two maybe afterwards but then need to get on.
They do not leave! I end up offering to make them a sandwich when I make our lunch, ooh lovely they said and settled in! The first week I was so taken aback I didn't say anything...got to school run time and I thought they'd go, nope, they said they'd stay to see eldest child 'for a bit' after school. They left when I put the children to bed at 7pm!
Thought it was maybe a one off but second week they didn't seem to be moving when it got to school run time, and again they stayed until bedtime. So then next week I told them they'd have to go when I left for school because we were going out after school. They stayed until school run time every week after that though. So two hours babysitting in the morning came at a cost of a four hour visit afterwards!
It was so awkward because they were doing me a favour by coming at all, and my children love them. They're really local and we see them all the time so they're not lacking grandparent hours! But it's like they'd decided 'Wednesdays are our day to be at Xs house' and that was that!
Once the course ended I thanked them for helping me out and MIL told me that they would continue coming 'for the day' on a Wednesday... erm no! I just said no need thank you, but that I would take youngest to them sometimes for a change (which I do probably once a week) so then I was in control of when we went home!